A power management circuit is a circuit which controls the power supply to at least one functional component of an electronic device. If a device comprises a plurality of functional components, a dedicated power management circuit may be associated to each of these functional components.
For example, a cellular terminal usually comprises a cellular modem as a first functional component, an application engine as a second functional component, etc. The term application engine refers to the “motor” of a terminal. It may comprise the application processor and related memory components, including the core software, hardware drivers, low level software and operating system, as well as related power management components and interfaces to peripherals like display, camera, keyboard, Bluetooth™ module, etc., even to the cellular modem. The application engine does not contain the mentioned peripherals, though. The idea behind the use of such an application engine is that it allows constructing many kinds of terminals using the same core components but varying for example the user interfaces, like display and keyboard. In general, it enables the construction of products that look quite different and have different features but that use the same technology inside.
Each of the functional components may be realized for instance on a separate chip, and each chip may comprise a power management circuit. Each power management circuit can be realized for instance in form of an Integrated Circuit (IC) or an Application Specific Integrated Circuit (ASIC).
In case a plurality of power management circuits are present, one of the circuits is normally defined to be a master circuit, while the other circuits are defined to be slave circuits. The master circuit controls the power-up and down of the entire system. This can be realized with an enable signal which is controlled by the master circuit and which can be detected by the slave circuits. Whenever the enable signal becomes active, the slave circuits cause the associated functional components to be powered up, and whenever the enable signal becomes inactive, the slave circuits cause the associated functional components to be powered down.
Typically, a power management circuit also comprises a thermal shut-down function. That is, in case the temperature of an associated functional component or of a chip comprising the associated functional component rises too high, the power management circuit forces the component to power off, in order to prevent a destruction of the component.
Abnormal heating may be caused by many reasons, for example by an overload on some regulator, a short circuit, high ambient temperature, or even a crash of the system software.
The temperature can be sensed by a sensor. Such a sensor is normally integrated on a single chip together with the power management circuit. Usually, such a sensor monitors two limits. The lower limit is a warning limit and the higher limit is the actual shut-down limit.
In case a sensor detects that the warning limit is exceeded, it informs an associated power management circuit that the chip temperature has started to rise too high. The power management circuit provides thereupon a warning to a system processor, for example in the form of an interrupt. The system processor will then turn the system off in a controlled manner. To this end, it requests the master power management circuit to shut down the system. The master circuit deactivates thereupon the enable signal and shuts down the power for the functional component or components associated to the master circuit. The slave circuits detect the deactivation of the enable signal and shut down the power for the components associated to the slave circuits.
In some situations, however, the monitored temperature may exceed the shut-down limit before the system processor is able to turn the system off in a controlled manner. For instance, in case of a particularly fast increase of temperature upon a sudden short circuit, etc., the software of the system processor might not have enough time to react. Moreover, the system processor might suffer a crash after which the software is no longer functional and thus fails to power down the system. In cases in which the monitored temperature rises very fast above the shutdown limit, the concerned power management circuit should therefore force the associated functional component autonomously to power down, before hardware damage occurs. In a possible implementation, for example, a power management circuit causes a shut down of an associated functional component, if an interrupt provided to the system processor is not acknowledged within a certain time limit.
Whenever there are multiple power management circuits in an electronic device, each of these power management circuits is capable of interrupting the system processor due to a detected rising temperature.
A problem may arise in a situation, in which a slave power management circuit detects an overheating, and in which the system software has crashed or is unable to respond to an interrupt by the slave circuit for some other reason, or the temperature rises so fast that the system software has not enough time to react.
If only one of the slave circuits detects a temperature rising above the shut-down limit, a situation may occur in which one component is switched off, while one or more other components remain switched on, that is, the power down of the system is not complete. In a cellular terminal, for example, it might happen that the cellular modem is still powered on, while the application engine has shut down. This, in turn, is a non-allowed system state, which can have unpredictable consequences and even result in hardware damage. At least a weird behavior might be seen by the user of the electronic device.